I'm considering insulating (and drywalling) my garage, which already has insulated garage doors. My main goal is to keep it from getting much below freezing during the winter.

This garage is located in the North Cascades (2000' elevation) in Washington state, with a typical yearly temperature range of 0F - 105F.

I'm not too worried about insulating the walls, but wonder: Do I need a vapor barrier?

For the ceiling (rafters 24" oc) would it be easiest to place normal batting between the rafters and then hang the drywall, or hang the drywall first and then lay the insulation afterwards? Or would blown-in insulation be better? Which is typically cheaper? And again, do I need a vapor barrier?

The space that will become the attic is vented via wall vents under the peak of the roof on both sides of the garage.

2 Answers
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I have a few questions and observations that will effect what you should do. First, I see that the peak (ridge) seems to be over the existing framed and drywalled wall. What is on the other side of this wall? Is this garage attached to the house or free standing? Do you have or intend to put any heat or heat dump register in this space?

I also noticed that the roof framing is standard trusses, not attic trusses. Where is the attic space going to be located? If the attic space is going to be over the bay you pictured, then you are limited in the amount of insulation you can install and still have a floor for storage. If the collar ties are 2X6, then the most blanket type insulation you can install is R19. 2X4= R11. Fiberglass blanket would definitely be the most cost effective in a small space such as you have. I would use unfaced F/G, and a 4 or 6 mil plastic vapor barrier installed before you drywall. As for installing drywall, you really should install strapping at a max of 16" on center to mount your 5/8" fire rated rock to. 24" inch on center is too large for hanging rock overhead, especially 5/8" rock. If garage is attached, 5/8" drywall is minimum code in most areas.

I also noticed that you have a metal roof. Metal roofs are great, but are notorious for condensation. This makes it absolutely necessary to have a good vapor barrier and good venting to avoid potential interior "rain storms" lol.

My other consideration would be insulating the exterior walls. Since there seems to be only one long wall and very little framed cavity wall on the gable ends, it would be advantageous to go ahead and finish the job by insulating and drywalling these walls. If you only do the ceiling, you will not have a good barrier to air, heat and humidity leaking by the ends of the ceiling at the wall plates and entering the attic area. This kinda defeats the whole idea of what you are doing.

Are there any other options for ceiling material besides 5/8" fire-rated rock? I have 24" on-center trusses in my attached garage and was considering finishing it as well. I actually would prefer painted tongue-and-groove, as I think it's easier to install.
– allnightgroceryMay 22 '11 at 12:31

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@Inkspeak. Yes there are alternatives. Just be sure if you are still going to use the space for cars, gasoline powered equipment, or any fuel fired appliances, that you check local codes for fire rated ( 1 hour) acceptable materials. In my opinion, T&G painted pine would scare the hell out me. A fire from a vehicle would turn any wood sheathing into a flaming infernal in minutes. Just my opinion, but I wouldn't install it for a customer unless it was over fire rated sheetrock, and then I'd be hesitant without a sprinkler system.
– shirlock homesMay 22 '11 at 13:18

I should be clear. Fire rated sheathing is only required on a common wall to a living space (in an open unfinished garage) as a minimum requirement of IRC. Many local jurisdictions have much stricter standards. Check with your local code enforcement officials.
– shirlock homesMay 22 '11 at 13:23

To answer some of your questions: - The garage is free-standing (not attached) - The previous owner used this property as a vacation rental, and walled in one bay of the garage (up to the trusses), presumably for personal storage. That's the wall you see in one of the pics. - I am planning on insulating and finishing the outside walls. I'm more curious about the right way to do the ceiling - I'm not concerned with adding a floor or any storage space in what will be the "attic", though I am planning to add pull-down stairs for access.
– user2446May 22 '11 at 21:11

I prefer fiberglass batting over blown-in insulation because it's not as messy, you don't have to rent a machine to do it so you can do it at your own pace, and if you need to do any subsequent work in the attic, it's easier to move it out of the way then move it back again later. I'm not sure about the relative cost, but I frequently see special offers for fiberglass insulation in HD and Lowe's so shop around.

If you choose fiberglass, I would install the insulation before the drywall, just because you'll have more room to work in.